In gas turbine engines, such as aircraft engines for example, air is drawn into the front of the engine, compressed by a shaft-mounted rotary-type compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft as the compressor. The flow of gas turns the turbine, which turns the shaft and drives the compressor and fan. The hot exhaust gases flow through the engine nozzle at the back of the engine, generating thrust to propel the aircraft forward.
During operation of gas turbine engines, the temperatures of combustion gases may exceed 1650° C. (3000° F.), considerably higher than the melting temperatures of the metal parts of the engine which are in contact with these gases. Operation of these engines at gas temperatures that are above the metal part melting temperatures is a well established art, and depends in part on supplying a cooling air to the outer surfaces of the metal parts through various methods. The metal parts of these engines that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are the metal parts forming combustors and parts located aft of the combustor.
The metal temperatures can be maintained below melting levels by using thermal barrier coatings (TBCs), often in combination with various cooling hole designs incorporated into some engine components. The TBC is typically applied to the component by a thermal spray process. However, the thermal spray process often results in overspray that partially or completely blocks the component's cooling holes. The percent blockage typically increases substantially as the thickness of the TBC grows.
As a result, present thermal spray and cleaning processes involve a multi-step, highly labor intensive process of applying a partial layer of TBC coating, allowing the component and the TBC to sufficiently cool to a temperature at which the component can easily be handled, removing the component from an application fixture on which the thermal spraying takes place, and removing any masking, which is then followed by separately removing the well-cooled, solidified coating from the cooling holes using a water jet or other cleaning methods. To prevent the cooling holes from becoming obstructed beyond a level from which they can be satisfactorily cleaned, only a fraction of the desired TBC thickness is applied prior to cleaning. As a result, the entire process must typically be repeated several times until the desired TBC thickness is reached. This complex process results in low productivity, high cycle time, and increases costs by a factor of five to ten times that of applying the same TBC to a similar non-holed part.
What is needed is a method for applying a TBC or other coating by a thermal spray process to an article having cooling holes and cleaning those holes concurrent with the thermal spray process to reduce time and monetary costs associated with current incremental, multi-stage coating and cleaning processes.